Early day running tracks were made of clay. These evolved into cinder and then crushed brick tracks. These were not all-weather surfaces. They offered little cushion to protect the athlete and no rebound to aid the runners. The most popular all-weather surface worldwide is an asphalt concrete street or road. Again, little or no cushion or rebound to the runner.
An asphalt concrete pavement roadway is constructed to carry loads with infinite load applications. Asphalt pavement is a general term applied to any pavement that has a surface constructed with an asphalt binder. Normally, it consists of a surface wearing course (layer) of mineral aggregate coated and cemented (bound) with asphalt; and one or more supporting courses. This asphalt pavement structure usually consists of three layers (courses).
Three layers (courses) are laid on a prepared subgrade. To get this subgrade to the proper line and grade, you will have cuts and fills. The cuts should be and are overcut or scarified usually a minimum of twelve inches. All of this fill and scarified material is brought to optimum (i.e., most favorable or best possible for a certain purpose) moisture content and is compacted to a certain uniform density. This prepared subgrade material is unbound material.
The first layer applied to the subgrade is the subbase. This subbase is usually granular material or selected soil. This layer is unbound and is normally not treated. It is, however, watered to optimum moisture content and compacted to a certain uniform density.
The second layer is the base course. This is compacted granular material (such as crushed rock, slag, gravel, sand, or a combination of such) or stabilized soil. Where it is an aggregate, it is referred to as aggregate base course, or simply ABC. This ABC is brought to optimum moisture content and compacted to a certain uniform density.
The third or top layer is the asphalt wearing surface. This is an asphalt bound aggregate mixture and is known as asphalt concrete. This asphalt concrete is usually applied in two or more lifts (layers or courses).
All of these layers are compacted to a very high percentage of their maximum density. Also, these layers are all bound securely together in accordance with conventional road engineering practices. This is to give this roadway the capability to support the load applications it is expected to receive. All of the research and development technology has evolved around this goal. Many generations of human beings have gone to this end.
Sometimes materials are used to treat or stabilize granular base and subbase materials or selected soils. These are Portland cement, lime, calcium chloride or salt (sodium chloride). In the case of adding Portland cement, it is added in lesser quantities than in Portland cement concrete. This turns these layers into a rigid material. This is sometimes referred to as soil cement. This makes this a bound layer.
Where the prime objective is to resist these load applications, they have come up with some very undesirable side effects, the major side effect being cracking. The transverse (across the roadway) cracking is the number one side effect in this three-layer road construction system. This transverse cracking is more predominant in cement treated aggregate base courses (usually called cement treated base or CTB).
This three-layer system has one very big built-in flaw which causes the transverse cracks in the top asphalt concrete layer. This flaw is reflection (or reflective) cracking.